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Venting for Outdoor Installations
The venting system is designed for direct discharge of flue gases
to the outdoors. The vent discharge opening should be located to
provide an unobstructed discharge to the outside and should be
located as far from the combustion air inlet as possible but in the
same pressure zone.
The vent duct should pitch down toward the outlet, to ensure that
any condensate that occurs in the vent duct drains away from the
combustion blower fan housing. The duct opening should be pro-
tected by a 1/2 in. x 1/2 in. (12 mm x 12 mm) mesh screen. An op-
tional rainhood may be used over the discharge opening to prevent
wind-driven rain from entering the vent duct, but should not inter-
sect the flue gas discharge path. See Fig. 145.
IMPORTANT: Outdoor units must be individually vented.
Venting For Indoor Installations
All duct furnaces must be connected to a venting system to convey
flue gases outside of the heated space. Vent systems must be sized
and installed in accordance with ANSI Z223.1 (NFPA 54) or in
Canada CAN/CGA - B149.
There are two methods for venting indoor furnace installations:
vertical venting and horizontal venting.
Vertically Vented Duct Furnaces (Category I)
Proper venting of the heating units is the installer's responsibility.
Vent piping is supplied by others. When operated with the venting
system in place, proper duct furnace operation must be verified,
including flue gas analysis of each connected furnace.
VENT HOOD
AHU
CABINET
Fig. 145 — Outdoor Horizontal Venting
Use single wall or double wall (Type B) vent pipe of diameters
listed in the following table.
INPUT RATING
(BTUH)
75,000 - 149,999
150,000 - 400,000
401,000 - 600,000
Maximize the height of the vertical run of vent pipe. A minimum
of 5 ft (1.5 m) of vertical pipe is required. The top of the vent pipe
must extend at least 2 ft (0.61 m) above the highest point on the
roof. (Use Listed Type B vent for external runs.) An approved
weatherproof vent cap must be installed to the vent termination.
Horizontal runs must not exceed 75% of the vertical height of the
vent pipe, up to a maximum of 10 ft (3 m). Horizontal runs should
be pitched upward
/
in. per ft (21 mm/m) and should be support-
1
4
ed at 3 ft (1 m) maximum intervals.
Design vent pipe runs to minimize the use of elbows. Each 90-de-
gree elbow is equivalent to 5 ft (1.5 m) of straight vent pipe run.
Vent pipe should not be run through unheated spaces. If such runs
cannot be avoided, insulate vent pipe to prevent condensation in-
side vent pipe. Insulation should be a minimum of
mm) thick, foil-faced material suitable for temperatures up to
500°F (260°C). Vent connectors serving Category 1 heaters must
not be connected into any portion of a mechanical draft system op-
erating under positive pressure. See Fig. 146.
IMPORTANT: Dampers must not be used in vent piping runs.
Spillage of flue gases into the occupied space could result.
VENT HOOD
FLUE GAS FLOW
UNOBSTRUCTED
COMBUSTION
AIR INLET
a39-4395
LOUVER
COMBUSTION AIR OPENING
190
VENT PIPE
INPUT RATING (W)
DIAMETER
21,980 - 43,958
5 in. (126 mm)
43,960 - 117,228
6 in. (152 mm)
117,229 - 175,842
7 in. (178 mm)
COMB.
BLWR.
AHU
CABINET
/
in. (12.7
1
2
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